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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
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Hi Greg: Can you send me the output from ncdump -h on a typical file ? How does the parallel part work? do certain sections of the file get reserved for different nodes, or is there a master node that essentially serializes the writing? Greg Sjaardema wrote:
As a quick answer to the question, we (Sandia Labs) use netcdf underneath our exodusII file format for storing finite element results data. If the mesh contains #nodes nodes and #elements elements, then there will be a dataset of the size #elements*8*4 (assuming a hex element with 8 nodes, 4 bytes/int) to store the nodal connectivity of each hex element in a group of elements (element block). Assuming 4GiB, this limits us to ~134 Million elements per element block (using CDF-2) which is large, but not enough to give us more than a few months breathing room. Using CDF-1 format, we top out at about 30 million elements or less which is hit routinely. There is a pdf file at http://endo.sandia.gov/SEACAS/Documentation/exodusII.pdf that shows (starting at page 177) how we map exodusII onto netcdf. There have been some changes since the report was written to reduce some of the dataset sizes. For example, we split the "coord" dataset into 3 separate datasets now and we also split the vals_nod_var into a single dataset per nodal variable. --Greg John Caron wrote:Hi Rob: Could you give use case(s) where the limits are being hit? I'd be interested in actual dimension sizes, number of variables, whether you are using a record dimension, etc. Robert Latham wrote:Hi Over in Parallel-NetCDF land we're running into users who find even the CDF-2 file format limitations, well, limiting. http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/NetCDF-64-bit-Offset-Format-Limitations.html http://www.unidata.ucar.edu/software/netcdf/docs/faq.html#Large%20File%20Support10 If we worked up a CDF-3 file format for parallel-netcdf (off the top of my head, maybe a 64 bit integer instead of an unsigned 32 bit integer could be used to describe variables), would the serial netcdf folks be interested, or are you looking to the new netcdf-4 format to take care of these limits? Thanks ==rob============================================================================== To unsubscribe netcdfgroup, visit: http://www.unidata.ucar.edu/mailing-list-delete-form.html ==============================================================================
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